1. Field of the Invention
The present invention relates to optical scanning apparatus and multi-beam optical scanning apparatus and, particularly, the invention is suitably applicable to image-forming apparatus, for example, such as laser beam printers involving the electrophotographic process, digital copiers, and the like, constructed so as to record image information by reflectively deflecting light from light source means by deflecting means to optically scan a surface to be scanned, via scanning optical means with the light.
2. Related Background Art
In the optical scanning apparatus such as the laser beam printers, the digital copiers, etc. heretofore, the image information was recorded in such a manner that the light optically modulated according to an image signal and outputted from the light source means was periodically deflected by the deflecting means which consisted of, for example, a polygon mirror, and was converged in a spot shape on a surface of a photosensitive recording medium by the scanning optical means with the fθ characteristics to optically scan the surface.
FIG. 13 is a schematic diagram to show the principal part of a conventional, optical scanning apparatus. In the same figure a diverging beam emitted from the light source means 91 is converted into a nearly parallel beam by a collimator lens 92 and the nearly parallel beam is restricted in the beam width by a stop 93 to enter a cylindrical lens 94 having a predetermined power only in the sub-scanning direction. The nearly parallel beam entering the cylindrical lens 94 emerges in the state of the nearly parallel beam in the main scanning section as it is. In the sub-scanning section the beam is converged to be focused into an almost linear image on a deflection facet (reflective surface) 95a of an optical deflector 95 consisting of a polygon mirror. Then the scanning optical means (fθ lens system) 96 with the fθ characteristics guides the beam reflectively deflected by the deflection facet 95a of the optical deflector 95, via a return mirror 98 to a surface of photosensitive drum 97 as a surface to be scanned. The optical deflector 95 is rotated at nearly equal angular velocity, whereby the beam scans the surface to be scanned 97 at almost constant speed to record the image information thereon.
To make the apparatus from the optical deflector 95 to the surface to be scanned 97 more compact, it is necessary to effect good correction for optical performance of the fθ lens 96 throughout wide angles of view. For example, Japanese Patent Application Laid-Open No. 7-113950 discloses an example of correction for curvature of field (image positions) in the sub-scanning direction and at wide angles of view by provision of only one surface wherein curvatures in the sagittal direction vary on an asymmetric basis with respect to the optical axis and wherein magnitude relations of curvatures in the sagittal direction are different on the upper and lower sides of the optical axis.
There was, however, the problem that nonuniformity of lateral magnification (which will also be referred to hereinafter as “sub-scanning magnification”) in the sub-scanning direction appeared prominent at wide angles of view and even if the image positions in the sub-scanning direction were corrected the spot size would vary in proportion to sub-scanning magnifications at respective scanning positions. Further, in the case of the optical scanning apparatus using multiple beams, they suffered from the problem that with deviation of the sub-scanning magnifications from a fixed value, line pitch intervals in the sub-scanning direction varied at every scanning position on the surface to be scanned during the optical scanning of that surface, so as to result in irregular pitch.
The scanning optical means needs to be located near the optical deflector in order to decrease the cost by decreasing the size of the lens. However, there was the problem that it increased the sub-scanning magnification and the asymmetry of the image positions in the sub-scanning direction and the asymmetry of the sub-scanning magnifications appeared more prominent.
An object of the present invention is to provide a compact, high-definition, optical scanning apparatus with wide angles of view capable of effecting good correction for curvature of field (image positions) in the sub-scanning direction and correction to keep the sub-scanning magnification at a fixed value, by constructing the scanning optical means of a plurality of sagittal asymmetric change surfaces and properly setting the shape of each lens.
Another object of the present invention is to provide a compact, high-definition, multi-beam optical scanning apparatus with wide angles of view capable of keeping line pitch intervals in the sub-scanning direction constant throughout the entire, effective scanning area, by constructing the scanning optical means of a plurality of sagittal asymmetric change surfaces and properly setting the shape of each lens.